Within the automotive industry particularly, there has long been a desire for a glass that changes transparency to adjust for conditions of brightness. Automotive glass has been formed from photochromic materials which automatically adjust their transparency with changes in the light intensity. However, these materials do not permit the user to change or adjust the degree of transparency, making them undesirable for widespread automotive use.
Alternatively, electrochromic materials are characterized by optically variable properties and therefore are suitable for use within an automobile. Windows formed from electrochromic materials permit the occupants to control the light transmission into the vehicle. Electrochromic glasses may be used for privacy considerations, or to control solar generated heat or glare within the automobile.
An electrochromic device reversibly colors to a shade, such as dark blue, and clears to transparency, by electrochemical reduction and oxidation when an appropriate voltage is applied to the electrochromic cell. The reversible electrochemical reactions occur at low operating voltages provided by an external power supply, and the changes in coloration occur over a relatively short period of time. Once the desired coloration is achieved, the electrochromic windows could maintain the same intensity of coloration without the aid of the external power supply.
The basic structure of such an electrochromic window is an electrochromic cell consisting of three types of thin film materials. First, a transparent electrically conductive film is provided on a transparent glass support to collect and generate the required current. The electrochromic cell requires two of these glass supports each having the transparent electrically conductive film. Second, an electrochromic film is provided on each of the transparent electrically conductive films to form the optoelectroactive elements of the electrochromic cell. Generally, electrochromic devices utilize two different, but complementary, electrochromic materials, typically tungsten oxide (WO.sub.3) with Prussian Blue MFe.sup.III [Fe.sup.II (CN).sub.6 ] where M is a metal ion, or tungsten oxide with polyaniline. Lastly, the cell is formed by providing a film of an ionically conductive electrolyte between the two electrochromic films. The ionically conductive electrolyte provides the ions for the electrochromic process. The selection of an appropriate ionically conductive electrolyte is critical to the performance of the electrochromic cell.
Many types of ionically conductive electrolytes have been proposed by the art for electrochromic devices. Complementary electrochromic cells of the tungsten oxide and Prussian Blue have been made with ionically conductive electrolytes, such as a Li.sup.+ doped propylene carbonate or a polyamp material. In addition, the polyamp electrolyte material has been used in a complementary electrochromic cell between an electrochromic film of tungsten oxide and a second electrochromic film of a derivative of benzyl polyaniline.
However, despite the many varieties of electrochromic devices now available, a significant shortcoming exists with regard to these devices. An electrochromic cell within a window in an automobile will experience a wide range of temperatures from well below 0.degree. C. to in excess of 100.degree. C. The known polymer electrolytes used throughout the art, fail either due to inadequate conductivity at the low temperature or instability at the high temperature.
Therefore what is needed is an electrochromic device for use in an automobile window which can satisfactorily function between the temperature extremes experienced during normal automotive use.